I. Field of the Invention
The present invention relates to a concentration detecting apparatus using a polarographic sensor to detect or measure concentrations of a plurality of gas components.
II. Description of the Prior Art
Steam (H.sub.2 O) and carbon dioxide gas (CO.sub.2) are produced by plants and animals, including man at various levels depending on activity. The measurement of H.sub.2 O and CO.sub.2 content is performed in plant cultivation, ethological/zoogeographic control and so on to provide information about the degree of activity of plants and animals. A plant cultivation system (vegetable cultivation system) for stably producing fresh, pesticide-free crops (mainly vegetables) all year around by an industrial means has received a great deal of attention. Such a vegetable cultivation system has been developed mainly in Europe and the United States of America. Ecological environments are artificially controlled in such a plant cultivation system to produce crops irrespective of changes in natural environments. According to this system, a great amount of energy must be inevitably used, so that a high yield must be obtained to satisfy the high energy consumption. For this reason, optimal plant cultivation conditions must be found, and ecological control must be performed to maintain these optimal conditions.
In such a plant cultivation system (i.e., factory), carbon dioxide gas has a high concentration of about 1,000 ppm. The level of sunshine, temperature, humidity, the content of carbon dioxide gas, the quality and quantity of water, air circulation, leaf temperature and so on are monitored by sensors, and are controlled by a computer.
In this field, concentrations of oxygen gas, steam and carbon dioxide gas must all be continuously monitored. For this purpose, demand for a compact, low-cost apparatus has arisen.
A multi-collector type mass analyzer or the like is used to measure ecosystem factors such as photosynthesis, respiration, evaporation and translocation which contribute to the growth of plants. This analyzer can highly precisely measure gas components such as carbon dioxide gas, oxygen gas and steam which are absorbed in and/or exhausted from the plants. However, the mass analyzer is large in size and expensive.
Another conventional system is employed to measure concentrations of a plurality of gases. Separate sensors are used to measure the concentrations of oxygen gas, carbon dioxide gas and steam. For example a limiting electric current type oxygen sensor is used to measure a concentration of oxygen gas; a thermal conduction type sensor is used to measure a concentration of carbon dioxide gas; and an absolute humidity sensor is used to measure a concentration of steam. According to this system, a plurality of sensors are required, resulting in high cost, a large installation space, heavy weight, and cumbersome manipulation.
In addition, when the concentrations of different gases are separately measured as described above, various types of carbon dioxide sensors may be used. However, none of them can satisfy the prescribed requirements. For example, a carbon dioxide gas measuring infrared ray analyzer has a large size and high cost, a solution absorbing type sensor has a slow response time and a sensor tube cannot perform continuous measurement.